1. Field of the Invention
The present invention relates to a wafer cleaning apparatus for cleaning wafers for use in manufacture of semiconductor devices, and more particularly, to an improved wafer cleaning apparatus adapted to clean wafers whose topmost layers are formed from different kinds of layers.
2. Description of the Related Art
One of a semiconductor manufacturing process employed to form a desired circuit pattern includes a step of sequentially stacking a plurality of layers of various kinds (for example, an oxide layer, a polysilicon layer, a nitride layer, metal layer and the like) on a wafer. When foreign particles are interposed between layers in the course of such stacking process, undesirable characteristics may occur. Thus, after forming a predetermined layer on a wafer and before performing a necessary process on the formed layer or stacking another layer thereon, it is necessary to clean the surface of the wafer to remove the foreign particles from the wafer. A wafer to be cleaned may have at its upper portion one or more layers of various types. In other words, a wafer may be provided with only one layer at its upper portion, or be provided with a plurality of layers stacked at the upper portion. In such a wafer, a xe2x80x9ctopmost surface of a waferxe2x80x9d represents a topmost surface of a topmost layer of the wafer.
A wafer cleaning apparatus, which is so-called a scrubber, is used in cleaning the surface of the wafer. The wafer cleaning apparatus, as shown in FIG. 1, includes a vacuum chuck 2 installed on a frame 1, for vacuum-adsorbing a wafer (W) and chucking the same, a cleaning solution spray nozzle 3 for spraying a cleaning solution such as deionized water toward the top surface of the wafer W chucked on the vacuum chuck 2, an arm 5 rotatable around the axis of a shaft 4 supported on the frame 1, and a brush 6 installed at a free end of the arm 5.
In a state where the wafer W to be cleaned is chucked by the vacuum chuck 2, while the wafer W is rotated by a predetermined rotating means (not shown) and a cleaning solution is sprayed onto the wafer W through the spray nozzle 3, the brush 6 is moved horizontally above the wafer W according to the rotation of the arm 5. Then, foreign particles on the wafer W are swept away by the cleaning solution sprayed onto the rotating wafer W and churned by the brush 6, to then be removed from the wafer W.
While the wafer W is cleaned, if the brush 6 is moved horizontally in contact with the top surface of the wafer W, a layer formed on the wafer W may be scratched by the brush 6, resulting in damage thereto. In other words, in the wafer cleaning process, a distance between a topmost surface (i.e., a topmost surface of a topmost layer) of the wafer and a lower end of the brush 6 should be maintained. If the brush 6 is in contact with the topmost surface of a topmost layer of the wafer, the topmost layer of the wafer may be damaged by the brush. Thus, the brush 6 is spaced apart from the top surface of the wafer W by an appropriate distance to clean the layer without causing any damage. The distance between the brush 6 and the top surface of the wafer W is different depending on the kind of layer facing the brush 6, that is, a topmost layer formed on the wafer W. For example, in the case where the topmost layer on the wafer W is a metal or polysilicon layer, in consideration of the fact that the hardness of the metal or polysilicon layer is high so as not to be easily damaged, it is necessary to make the distance of the brush to the layer small to attain the maximum cleaning effect. The distance between the topmost surface (i.e., the topmost surface of the metal or polysilicon layer) of the wafer and the lower end of the brush may be, for example, less than 0.05 micro meters. Also, in the case where the topmost layer on the wafer W is an oxide layer, which is relatively soft and can be easily damaged, it is necessary to make the distance sufficiently large to the extent that the cleaning effect is ensured. The distance between the topmost surface (i.e., the topmost surface of the oxide layer) of the wafer and the lower end of the brush may be, for example, about 0.1 micro meters. Likewise, in the case where the topmost layer on the wafer W is a polysilicon layer or a nitride layer, the distance must be set in consideration of hardness of each layer and the cleaning effect. The numerical values of the distance may differ depending on, for example, characteristics of a cleaning apparatus.
The distance between the top surface of the wafer W and the lower end of the brush 6 can be changed by adjusting the height at which the brush 6 is installed with respect to the wafer W chucked by the vacuum chuck 2. However, the adjustment of the height of the brush 6 requires much time and endeavors and high cost, in terms of the repeated cleaning operation of sample wafers and the quality test of cleaned sample wafers. Therefore, in the case of cleaning a plurality of wafers having layers of various kinds at their topmost layers, several wafer cleaning apparatuses each of which is dedicated to a specific one of the layers are used in actual manufacturing fields, rather than adjusting the height of the brush 6 according to the kind of the layer formed on the topmost layer of the wafer W. For example, in the case where four layers, such as an oxide layer, a polysilicon layer, a nitride layer and a metal layer, are sequentially stacked on a wafer W, four wafer cleaning apparatuses are provided which are dedicated to the four layers, respectively. A distance between a brush of each wafer cleaning apparatus and the top surface of a wafer to be cleaned by the wafer cleaning apparatus is set in consideration of the hardness of the topmost layer of the wafer.
However, if the number of wafer cleaning apparatuses needed to clean wafers become the same as the number of different layers to be stacked on a wafer, it may be prohibitively costly to prepare many wafer cleaning apparatuses if the wafer has many layers. Also, the time and costs for installing the wafer cleaning apparatuses increases and a wide space is required for the installation of the wafer cleaning apparatuses.
To solve the above and other problems, it is an objective of the present invention to provide an improved wafer cleaning apparatus which is adapted to clean a plurality of wafers whose topmost layers are different kinds of layers, whereby the cost for providing a wafer cleaning apparatus of the present invention is less than that of a conventional wafer cleaning apparatus, and space required for installing a wafer cleaning apparatus may be reduced.
To accomplish the above objective of the present invention, there is provided a wafer cleaning apparatus includes a chuck for chucking a wafer to be cleaned, means for rotating the wafer chucked by the chuck, a cleaning solution spray nozzle for spraying a cleaning solution toward a surface of the wafer rotated by the rotating means, at least two brushes installed to be moved horizontally above the wafer with a predetermined distance spaced apart from the surface of the wafer in a contact state with the cleaning solution sprayed on the surface of the wafer, and brush moving means for selectively moving the respective brushes horizontally above the wafer, wherein distances between the surface of the wafer and lower ends of the respective brushes are different from each other.